The goals are:
* To have better result in the GetCapabilities URL without having to fill it in the Project configuration
* Be able to be set by an integrator with environment variable or by providing headers from a proxy
The server resolution will use the following order:
- Value defined in the project per service.
- The ``<service>_SERVICE_URL`` environment variable.
- The ``SERVICE_URL`` environment variable.
- The custom ``X-Qgis-<service>-Servcie-Url`` header.
- The custom ``X-Qgis-Service-Url`` header.
- Build form the standard ``Forwarded`` header.
- Build form the pseudo standard ``X-Forwarded-Host`` and ``X-Forwarded-Proto`` headers.
- Build form the standard ``Host`` header and the server protocol.
- Build form the server name and the server protocol.
